In the construction of projection objectives with relatively short Gauss focal length, e.g. for portable overhead projectors, the conventional method has been to make the objective with a small aperture, in order to obtain with few lenses, a generally acceptable image quality. An optical system is thus afforded which can be produced at relatively low costs and which has relatively small dimensions and low weight, so that the condition of portability is satisfied, but with the disadvantage that such an optical system at best only allows acceptable projection of very highly luminous objects such as transparencies or the like.
An attempt to make an objective allowing acceptable image quality and light strength, even when projecting objects of relatively small luminosity, is disclosed in the Swedish Pat. No. 7407134-1. Such an objective includes a modified triplet supplemented by a large positive lens a long way in front of the triplet. This kind of objective has indeed a large aperture, and affords acceptable image quality, but as indicated, it requires four large lens elements and the Gauss focal length needs to be about 600 mm, in order that the projector operator can manipulate the image of the object. Due to the necessary size and weight of the objective, as well as the large number of lenses necessary for it, this previously known objective is neither economically acceptable nor compatible with the portability requirement.
The conviction of those skilled in the art has been that a proper projection objective must include a plurality of lens elements, even if some lens surface has been given an aspheric correction. The technique of aspherically correcting a lens surface for reducing certain image reproduction errors or aberrations is known, but is commercially limited, since the cost of producing such lenses is relatively high. It has therefore appeared as being more economically favourable in the construction of a projector objective to increase the number of lens elements therein, if it is desired to improve the resulting image quality, instead of aspherically correcting any of the lens surfaces in the objective since, as mentioned, such persons were convinced that the objective necessarily must include a plurality of lens elements in order to avoid the most serious aberrations.